1. Technical Field
The invention concerns a magnetically enhanced reactive ion etch (MERIE) plasma reactor and specifically an MERIE plasma reactor having a shallow magnetic field with minimal strength at the wafer surface and with radial symmetry across the wafer surface.
2. Background Art
A plasma reactor for processing a substrate such as a semiconductor wafer typically includes a reactor chamber containing a processing gas and a pedestal supporting the wafer within the chamber. In order to ignite a plasma by ionizing the processing gas, a radio frequency (RF) power source is applied to the wafer pedestal. The RF power coupled into the chamber ignites and maintains a plasma while also attracting ions toward the wafer pedestal. The RF power excites the electrons in the chamber by reason of their high charge-to-mass ratio, and the excited electrons collide with neutral species (e.g., molecules and radicals) of the processing gas to create ions. The ions react with the wafer on the pedestal to etch certain thin films thereon, for example, while some ion-wafer collisions cause ion bombardment or sputtering damage on the wafer surface, depending upon the energy with which the ions collide with the wafer surface. The ion energy is determined by the RF power applied to the wafer pedestal.
A fundamental limitation of such a plasma reactor is that there is a tradeoff between the plasma density and the ion bombardment damage on the wafer. This is because in order to increase the plasma ion density in the chamber, the RF power applied to the wafer pedestal must be increased, which in turn increases the ion energy at the wafer surface, thereby increasing ion bombardment damage. This tradeoff limits the performance of a plasma reactor. In a plasma reactor which performs a plasma etch process, the processing gas is an etchant such as HF, CHF.sub.3, CF.sub.3 or CF.sub.6, for example, and the etch rate is determined in large part by the density of fluorine-containing ions, which in turn is limited by constraining the RF power applied to the wafer pedestal to avoid excessive ion bombardment damage to the wafer.
Another problem is that increasing the RF power (in order to increase plasma ion density) increases sputtering of the chamber ceiling and walls. Such increased sputtering of the chamber ceiling and walls increases the amount of sputtered material introduced into the chamber and onto the wafer which can interfere with the etch process. For example, if the ceiling is quartz and the process being performed on the wafer is a silicon dioxide etch process, then some fraction of the quartz sputtered from the ceiling deposits onto the wafer surface and competes with the etch process, thereby reducing the etch rate. Also, unless the ceiling is of a material compatible with the particular etch being carried out on the wafer (e.g., a silicon dioxide or quartz ceiling for a silicon dioxide etch process, or an aluminum ceiling for an aluminum etch process), then the ceiling material sputtered into the chamber contaminates the wafer. (Sputtering of the ceiling also consumes an expensive component of the chamber). As another example, if the processing gas includes CF.sub.3 or CF.sub.6, the polymer material formed therefrom on the interior ceiling surface may be sputtered onto the wafer surface, thereby contaminating microelectronic devices on the wafer surface.
One technique for enhancing the plasma ion density in a plasma etch process without necessarily increasing ion bombardment damage on the wafer and sputtering is magnetically enhanced reactive ion etching (MERIE). In this technique, the plasma reactor described above is improved by the addition of plural (typically four) ring magnets placed symmetrically around the sides of the chamber. Typically, the diameter of each magnet is on the order of the height of the reactor chamber. The MERIE magnetic field produced within the chamber by these magnets causes the electrons--due to their large charge-to-mass ratio--to assume a complex circular and spiral motion in addition to their vertical linear motion induced by the RF power applied to the wafer pedestal. The circular and spiral motion of the electrons induced by the MERIE magnetic field increases the ionizing collisions by the electrons, thereby increasing the plasma ion density. The result is that ion density--and therefore the etch rate--is increased. However, there is no proportionate increase in ion bombardment damage on the wafer because the RF power applied to the wafer pedestal is not increased. Moreover, any spiral motion of the ions induced by the MERIE magnetic field does not directly increase the ion-wafer collision energy.
The ion-wafer collision energy is not proportionately increased by the MERIE magnetic field because any spiral motion of the ions induced by the MERIE magnetic field is generally in a horizontal plane (parallel to the wafer surface).
One problem with such MERIE techniques is that the magnetic field within the chamber is necessarily discontinuous because it is produced by (four) discrete adjacent magnets. The plasma ions tend to be focused into "corner" areas between the magnets, thus producing non-uniform etch rates across the wafer surface, a significant disadvantage. Further, it has been generally found that the plasma ions tend to migrate toward the wafer edge and away from the wafer center, thus contributing to the non-uniformity in etch rate across the wafer surface. Often, the etch rate tends to vary greatly along a given radius of the wafer, the etch rate on the wafer increasing with the radius.
This problem of non-uniform etch rate has been ameliorated in MERIE plasma reactors by rotating the magnetic field produced by the plural (four) magnets placed around the side of the chamber. Typically, such rotation is achieved by employing electromagnets around the sides of the chamber and applying RF signals in the magnet windings, a sine wave RF signal being applied to the windings of alternate ones of the magnets and a cosine wave RF signal of the same frequency being applied to the windings of the remaining ones of the magnets, for example. The idea is that the rotation of the magnetic field within the chamber across the wafer surface disperses corner effects and reduces ion focusing and thereby reduces plasma ion non-uniformity across the wafer surface, thus providing a partial solution to the problem.
However, this solution raises two additional problems. First, the rotation of the MERIE magnetic field must be limited to below ten Hertz to avoid excessive heating of the chamber side walls, thus limiting its efficacy. Second, the rotation of the MERIE magnetic field across the wafer surface produces charge damage in the microelectronic semiconductor structures already fabricated on the wafer surface. For example, thin gate oxide layers are particularly susceptible to breakdown. Such breakdown occurs because the changing magnetic field across the wafer surface produces relatively large forces on charges already accumulated in such microelectronic structures during plasma processing in the reactor.
Even without rotation of the magnetic field there is a risk of charge damage to microelectronic structures simply because the magnetic field itself is non-uniform across the wafer surface, leading to non-uniform plasma ion and electron density across the wafer surface. Such non-uniform plasma ion and electron distribution across the wafer surface leads to non-uniform charge accumulation and electrical potential differences across the wafer surface. Such potential differences can be sufficient to break down the more susceptible features (e.g., thin gate oxide layers) of the microelectronic structures on the wafer. Accordingly, the inventors herein recognize that the presence of the MERIE magnetic field at the wafer surface is itself a problem which limits the performance of MERIE reactors.
Therefore, it is an object of the present invention to provide an MERIE magnetic field which is of a minimum strength at the wafer surface and of maximum strength elsewhere in the reactor chamber.
It is another object of the present invention to provide an MERIE magnetic field which has improved uniformity across the wafer surface.
It is a related object of the present invention to provide an MERIE magnetic field which has radial symmetry across the wafer surface.